In the field of controllable solid propellant rocket motors which employ extinguishable propellants, additives have been used to aid extinguishment, and to suppress spontaneous reignition of the propellant. Extinguishable propellants are capable of undergoing extinguishment when the motor pressure is rapidly reduced. Such extinguishment can be made to occur before all of the propellant has been consumed.
Propellants intended for controllable motor application must possess certain properties that are not normally desirable in conventional solid propellants. These include a high burning rate exponent (n); a high critical pressure (Pcr), below which the motor cannot sustain steady-state operating pressure; a high pressure deflagration pressure (Pdl), below which combustion will not occur; and a high threshold ignition pressure (Pth), below which propellant reignition will not occur upon exposure to a given heat flux.
In addition to these special properties, the ballistic and mechanical properties of current state-of-the-art propellants are wanted. These properties include: ease and economy of manufacture, high specific impulse, mechanical properties suited to case bonding, adequate insensitivity and stability characteristics, and good aging characteristics.
Extinguishable propellants fall into three general classes: (1) those containing conventional composite binders and low energy oxidizers, such as, potassium perchlorate and flame suppressants, such as, sodium fluoride; (2) composite propellants using fluorocarbon binders; and (3) conventional composite or double-base binders filled with high contents of nitramine-type oxidizers, such as, RDX and HMX.
The conventional composite propellants containing KClO.sub.4 are of low impulse and produce large concentrations of alkali metal ions--and, as a consequence, a significant radar signature in their exhausts. If their specific impulse is improved through increased solids loading, standard processing techniques cannot be used.
The fluorocarbon propellant systems use expensive, non-commercially-available ingredients; have not been adequately characterized with regard to mechanical properties, and current fluorocarbon systems of developmental interest exhibit low elongation and poor case bondability.
The prior art technology has generally relied on the use of low energy oxidizers (KClO.sub.4), flame suppressants (NaFl), and coolants (dihydroxyglyoxime), oxamide, ammonium dihydrogen phosphate, ammonium tetrahydrogen metaphosphate, ammonium hexafluorophosphate, etc. to achieve extinguishment in controllable propellant motor applications.
In my copending application titled: "Fire-Retardant Insulation for Rocket Motors," Ser. No. 563,427, filed Mar. 31, 1975, it was disclosed that tetrabromophthalic anhydride (TBPA) and tetrabromophthalimide (TBPI), their salts and derivatives (when employed either as additives to the inert components when these inert components are being compounded or as crosslinking agents for these inert components) impart fire-retardancy characteristics to these inert components (liner, insulation, slivers, etc.). These components undergo afterburning (combustion of the gases which are produced by the pyrolysis of the inert components with the air ingested into the rocket motor due to the free convective circulation after motor burnout). The fire-retardance characteristics desired for the inert components was achieved by employing from about 10 to about 20 weight percent of the TBPA or TBPI.
The advantages of a reignition suppressant additive for use in solid, extinguishable propellants would be quite attractive if the desirable ballistic properties would not be offset. These suppressants involve a new mechanism for extinguishment, i.e., sweeping away of the combustible exhaust products from the propellant surface by non-combustible gases.
Therefore, an object of this invention is to provide an improved controllable propellant composition that includes a reignition suppressant which effectively raises the pressure deflagration limit, and which provides a source of non-combustible gases for sweeping away of combustible exhaust products from an extinguished propellant.